A few years later, in Men in Black II (2002), the neuralyzers have apparently been upgraded or modified, and now use a blue flash.

In Men in Black III (2012), a 1969-time-travel scene shows a young Agent K using a neuralyzer powered by a belt-mounted battery pack.

Agent K originally retired at the end of Men in Black, during the age of red neuralyzers. Yet at the end of Men in Black II, K is shown activating a blue neuralyzer located in the torch of the Statue of Liberty, using his watch.

The implication seems very strong that this neuralyzer was K's secret, as it is activated by a watch which he kept in the locker, with the key hidden across town.

So how could the Statue's neuralyzer be blue? Wouldn't it be red, as that was the last "version" of neuralyzer he used before retiring in 1997?

Ehhh, just cause they were upgraded doesn't mean that one couldn't be upgraded with the remote trigger left alone. And just because it was a surprise to J, doesn't mean the rest of the agency didn't know about it
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TylerShadsDec 29 '12 at 6:24

@TylerShads - In that case, wouldn't they have known about the watch? Why the hidden key? Would they really decommission K at the end of the first film without ascertaining the location of the trigger for such a massive neuralyzer? If not, that whole subplot is pointless. I still say it seems like his secret.
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UnsignedApr 29 '13 at 3:38

maybe he had a secret switch, and not the only switch. Maybe J was surprised because K remotely activated the neuralizer? J knew about it, but was surprised at K having a switch whole to himself. I think a neuralizer that size would have its controls at MIB HQ.
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kicker86Aug 15 '13 at 15:58

2 Answers
2

It is never implied in the movie whether the color of the neuralizers are any indication of their technological progress. You can simply put them as having different colors without reading too much into it.

There are many possibilities to your doubt, but I maintain that the colors were simply design choices, and nothing conscious. According to you, the mass neuralizer on the Statue of Liberty was blue. It could simply have been upgraded (if that is indeed the case) along with other neuralizer technology. It isn't shown that only K had the switch to activate the neuralizer. Maybe the MIB had a switch, and K personally had one? These details are not available in any canon. Hence, it will raise only opinions and speculations.

I honestly wouldn't read too much into the color change matter, as the neuralizer itself is really a sort of MacGuffin. They aren't the main driving point of the movie like an actual MacGuffin, but they exist simply to provide an explanation to the question, "What about the people who witness aliens or the problems they cause while on Earth?".

We're never really told exactly how the neuralizers work (if I recall correctly), only that they're used to wipe the memories of anyone who has witnessed aliens that shouldn't have. As such, their existence in the movie is merely a simple solution to what would otherwise be a complex answer, and the color change between them could possibly just be a continuity error.